1. Field of the Invention
This invention relates to a spacecraft capable of orbital flight in two alternative flight modes. The invention also relates to an attitude control method for controlling the orbital flight of the spacecraft in two alternative flight modes.
2. Description of the Prior Art
Various systems have been proposed for controlling attitude of an orbiting spacecraft, such as a satellite, with respect to the earth or the sun during its orbital flight. These systems fall generally into two categories, known as active attitude control systems and passive attitude control systems. Active attitude control systems include devices such as control moment gyroscopes, momentum wheels, thrusters, and magnetic torquers. Passive attitude control methods may include spin stabilization, gravity gradient stabilization, and magnetic field stabilization.
Gravity gradient stabilization has been used effectively to maintain an antenna or other instrument on a satellite pointing toward the earth. The principle of gravity gradient stabilization can be understood by considering the attitude motion of a satellite in the shape of a dumbbell consisting of two equal masses separated by a rod. Since the force of gravity is inversely proportional to the square of the distance from the center of the earth, the mass nearest to the center is attracted a little more strongly than the mass further away. This gradient in the gravitational field produces a torque tending to align the dumbbell with the local vertical. A deflection of the dumbbell away from the local vertical causes a restoring torque to be generated by the imbalance of the forces acting on the equal masses. The centrigugal force on one would be greater than the gravitational force on it, because these two forces are only equal at the center of mass. By contrast, the gravitational force on the other mass is greater than the centrifugal force on it, thus creating a net torque which forces the masses toward an alignment in the local vertical orientation. A horizontal orientation of the dumbbell is an unstable state of equilibrium. Although the forces on each mass are nominally equal, a slight rotation immediately increases the gravitational attraction of the lower mass and decreases the gravitational attraction on the upper mass. At the other extreme, a vertical orientation is a stable state of equilibrium because the difference in attraction for the two masses is at a maximum. Thus, the force of gravity will cause the axis of minimum moment of inertia of the spacecraft to align with the local vertical and point toward the earth or other body about which the spacecraft is orbiting. It is also true that when one axis of the spacecraft possesses the maximum moment of inertia, the dynamics of orbital motion causes that axis of the spacecraft to align normal to the orbital plane.
A number of systems have been proposed for the gravity gradient stabilization of satellites. In all of these systems, a rod-or boom-like structure extends outwardly from the satellite body. The rods or booms are usually of the type which are extendible after the satellite is in orbit and may be articulated to permit angular movement thereof. A weight is normally located at the end of the rod or boom. All of these rod and boom structures are intended to produce a satellite having a more dumbbell-like configuration in which the axis of the satellite along which the rod or boom lies will tend to align with the local vertical. In order to obtain structures which will exhibit the greatest amount of gravity gradient stability, the booms or rods are fully extended and are maintained in that position.
It has been found in a number of these gravity gradient stabilization techniques that the gravity gradient device tends to librate about the local vertical. Various dampers have been suggested as a means for controlling the librations of the gravity gradient devices. Since such damping devices are not universally effective, active attitude control devices have been suggested for use in connection with the passive gravity gradient stabilization techniques to damp the librations. However, applications of active attitude control devices for damping such librations are generally smaller, less costly, and more reliable than for applications where both attitude positioning and damping is required.
All of the prior art techniques for gravity gradient stabilization of orbiting satellites are designed to maintain the axis of the satellite having the least moment of inertia aligned with the local vertical and always pointing toward the earth during its orbital flight. Because these techniques were designed solely for an orbital flight mode in which an antenna or other instrument would always be facing toward the earth, they do not provide means for operating a satellite in a different orbital flight mode in which it would be oriented so that it faced in a direction other than toward the earth.